This invention relates to a new and improved process for recovering the aromatic components from aromatic containing feedstocks and is particularly adapted to hydrocarbon mixtures containing large concentrations of aromatics. More specifically, this invention is directed to an improved extraction process for the recovery of aromatics whereby selective solvents are employed. The extraction step is a simple countercurrent system, with an impure extract being recovered from the solvent and used as feedstock for a hydrodealkylation process.
Ordinarily when a solvent selective for aromatics is used for the purpose of isolating aromatics at high recovery (about 99%) and high purity (about 99%) the feed is first extracted with the solvent and then some device such as a countersolvent, temperature induced back wash, flashed back wash, or extractively stripped back wash is used to improve the aromatic purity obtained by the simple extraction step. The liquid feed may be contacted with a sulfolane solvent in an absorber-stripper column wherein the aromatics are removed as bottoms at high purity, but only at moderately high recovery (about 80 to 90%).
The combined use of solvent extraction and hydrodealkylation steps for aromatics production is a known combination; however, the prior processes involved intermediate purification steps to produce pure aromatics feed to the hydrodealkylation step. In U.S. Pat. No. 3,159,567, a solvent-extracted gas oil is purified to produce methyl naphthalene for hydrodealkylation. In U.S. Pat. No. 3,726,789, the aromatic components of a reformate stream are extracted and separated into high purity toluene for hydrodealkylation and also high purity benzene, xylene, etc. is recovered from the extract. The combination of solvent extraction and dealkylation is also shown in U.S. Pat. No. 3,213,152.
Prior workers have extended great efforts in producing a pure paraffin-free aromatic extract stream. It has been recognized that selective solvents which are used in extracting aromatic components from paraffins are not totally selective in rejecting the paraffinic hydrocarbons. This has led to prevalent use of a back wash step wherein a light paraffin stream is contacted with the solvent extract phase to minimize absorption of heavier paraffins, which are difficult to separate from the aromatic product downstream. Representative backwashing techniques are described in U.S. Pat. No. 2,877,173, 2,921,015, 2,938,858, 3,146,190, 3,222,416 and 3,435,087. All of these processes have the disadvantage of requiring light paraffin stripping to obtain adequate product purity. The recovered aromatic stream from such prior process is very pure, usually with no detectable paraffin (i.e., less than 0.01%), however, the energy requirements are high, due to the additional separation of recycled paraffins.
While it is known to employ simple countercurrent extraction of a reformate paraffinic-aromatic mixture without light paraffin back wash, the amount of impurity in the extract would be prohibitive for many end uses of the product. If the product specifications are not demanding, as in the case of gasolines blended with extracted aromatics in U.S. Pat. No. 2 956 006, a simple continuous extraction column without raffinate refluxing, back wash or other recycle expedient is feasible.